Adaptive mirrors permit distorted wave fronts to be reformed into undistorted wave fronts. An example of this problem occurs when a plain wave front from a distant star passes through the earth's atmosphere and is distorted by turbulent layers of air. The heating and cooling of the atmosphere by local weather effects cause these turbulent layers. In general, the further the light travels through the air and the denser the air is, the greater the amplitude of the distortion. Adjusting a mirror surface to match this distortion allows a reflected plain wave front to be observed. The actuator for adjusting the mirror surface to match the wave front distortion must operate very rapidly with response times of one thousandth of a second or less and is called an adaptive actuator. With an adaptive actuator, the adaptive mirrors should perfectly match the distorted wave front laterally and have half the amplitude of the wave front distortion. Another kind of actuator, called an active actuator, corrects for quasistatic surface errors in the mirror. Such errors may arise from inadequate polishing of the mirror, the force of gravity particularly as the mirror is tilted, unequal expansion of the mirror as a result of temperature changes or creep in the mirror surface as a result of internal strains in the mirror. The adaptive mirror should perfectly match these errors.
The phase of the light depends on the wavelength, so the shorter the wavelength the greater the phase error becomes when expressed in fractions of a wavelength.
The actuator correction for these faults does not need to have a rapid response time, but should be capable of being set very accurately. The shorter the wavelength, the greater the phase error and the more critical the required correction.
It is known in the art to use the Fried (freed) coefficient as a statistical measure of the phase error. As the Fried coefficient becomes smaller the distortion becomes greater.
As the light to be refocused moves from the infrared range to the visible range, the adaptive mirror surfaces which needs to be controlled to a fraction of a wavelength becomes subject to even finer tuning.
The stiffness of a composite mirror can be calculated by the structural stiffness module. The manner of this calculation may be found in “Development of Lightweight Mirror Elements for the Euro 50 Mirrors,” by Bennett et al. Proceedings of the 2nd Backaskog Workshop on Extremely Large Telescopes, Sep. 11-12, 2003, SPIE (in press).
Many piezoelectric materials are known. They have been made into actuators to move or displace upon application of a predetermined voltage. The voltage causes a piezoelectric substance to expend or contract. For a given voltage a single actuator of piezoelectric material expands in all directions. Anything connected to such a device is displaced or thrown this change in distance. For a given device a set throw range is established. If double the throw distance is needed two identical devices are placed together in electrical series connections or stacked. Applied voltage must be doubled for both actuators to fully respond and give double the throw distance. Lateral movement in such stacks is ignored. Third, fourth and more actuators are added to the stack for greater throw distances.
For audio devices piezoelectric material is coated on a sheet of metal, such as brass, steel, or other desired material creating the equivalent of a bimetallic strip. In this application the lateral expansion causes the device to bow to a given radius of curvature for a preselected voltage and thickness of the metal sheet and piezoelectric coating. In general, the thinner the greater the amount of curvature or bowing. These devices have been used to generate sound waves as the device bows and flattens.